The present invention relates generally to optical communication systems and more particularly to a system and apparatus for adding/dropping optical channels within a communications network.
Wavelength Division Multiplexing (WDM) is used to transmit a plurality of optical channels via an optical waveguide medium where each channel carries information signals within a network or system. Each channel within the multiplexed signal is associated with a particular wavelength, thereby increasing the information capacity of fiber optic systems. Such optical systems include, for example, local area networks (LANs), telecommunication systems and cable television systems.
In WDM and dense WDM (DWDM) systems, it is beneficial to select a particular wavelength from the group of wavelengths comprising the multiplexed optical signal. This is advantageous in order to drop and add the same or different channel wavelengths at various points within an optical network. Optical add-drop multiplexers are employed in telecommunications networks in order to add/drop one or more of these channels. When fiber Bragg gratings are used as the principal drop filtering elements in add-drop multiplexers, the drop channels are restricted to the shortest wavelengths within the channel plan to avoid radiation and/or cladding mode loss on the short wavelength side of the gratings. A more detailed explanation of the use of fiber Bragg gratings in optical add-drop multiplexers as well as the impact of radiation mode loss on the selection of add/drop channels can be found in U.S. Pat. Nos. 5,748,349 and 5,982,518 assigned to the assignee of the present invention and incorporated herein by reference.
With the advent of increasing channel counts in WDM networks, expandable systems have been introduced (e.g. see U.S. Pat. No. 5,504,609 assigned to the assignee of the present invention) where additional channels may be added to accommodate increasing bandwidth demands on existing networks. FIG. 1 schematically illustrates a channel plan for wavelengths within an optical communication system. A first group of channels 5 having wavelengths xcexi . . . xcexj can comprise, for example, 8, 16, 32, 40 or even more wavelengths in the 1.5 xcexcm range. With increasing channel counts, channel spacings decrease from about 200 GHz or more down to approximately 25 GHz or less. A first channel subset, on the short side of channel group 5, denoted as group 5xe2x80x2, is typically designated as the add/drop channels within the channel plan to avoid the radiation mode loss problem referenced above. The designated add/drop channels have associated with them particular transmission and filtering components within the WDM network tuned to transmit, and add/drop these particular channels. When bandwidth requirements necessitate additional channels to be added to the channel plan, it is advantageous not to add these channels so as to affect the drop channels and their associated components. For example, a second group of channels 6 (e.g. 8, 16, 32, 40, etc. additional wavelengths) can be added on the short side of channel group 5, on the high side of channel group 5, or a combination thereof. If channel group 6 or a portion thereof is added to the short side of channel group 5, channel group 5xe2x80x2 designated as the add/drop channels are no longer on the short wavelength side of the channel plan and therefore affect the radiation mode loss problem associated with the filtering elements. Moreover, particular components within the network are still configured to add/drop the channels within group 5xe2x80x2. Accordingly, to change these components would be costly and disadvantageous to the modular features associated with optical transmission systems. Additionally, when Bragg gratings are used, signal attenuation problems associated with radiation mode loss of the gratings can affect the through channels.
Thus, there is a need for an optical device which allows add/drop channel configurations to be maintained with existing network components while accommodating the addition of more channels to expand transmission capabilities. There is a further need for an optical add/drop multiplexer which separates a multiplexed optical signal into separate channel bands to accommodate add/drop channel selection.
The invention meets these needs and avoids the above-referenced drawbacks by providing an optical device for use in a wavelength division multiplexed optical communication system. A wavelength division multiplexed optical signal having a plurality of optical channels is supplied to an optical bandsplitter. The bandsplitter is configured to separate the plurality of optical channels into a first group of channels and a second group of channels. A first transmission path optically communicates with the bandsplitter and carries the optical channels associated with the first group of channels. One or more filtering elements positioned along the first transmission path suppresses transmission of a first set of channels within the first channel group corresponding to optical channels to be added to the multiplexed optical signal. The filtering elements are configured to transmit through channels corresponding to channels within the first group of channel which fall of the first set of optical channels. A second transmission path optically communicating with the bandsplitter carries optical channels associated with the second group of channels. An optical combiner optically communicates with the first and second transmission paths. The combining element is configured to output the through channels and the channels within the second group of channels.